Dynamics reverse osmosis membranes of ultrathin discs

ABSTRACT

Ultrathin membrane discs are prepared by spraying a casting solution of the membrane material onto a surface to form a number of individual discs, allowing the solvent to evaporate from the discs, and removing the discs from the casting surface. Slurries of these discs may be used to form dynamic membranes suitable for use in reverse osmosis applications such as the desalination of water.

United States Patent] [1 1 King Oct. 9, 1973 1 DYNAMICS REVERSE OSMOSIS3,674,152 1/1972 Manjikian 210/321 MEMBRANES 0F ULTRATHIN DISCS3,580,841 5/1971 Cndotte et a1. 210/23 inventor: William M. King,Diamond Bar,

Calif.

The United States of America as represented by the Secretary of theInterior, Washington, DC.

Filed: Mar. 28, 1972 Appl. N0.: 238,906

Assignee:

US. Cl 210/23, 210/321, 210/490, 210/500 int. Cl 15014! 13/00 Field ofSearch 210/22, 23, 321, 210/490, 500

References Cited UNITED STATES PATENTS W00 W00 et al. 210/500 X PrimaryExaminer-Frank A. Spear, Jr. Art0rneyEmest S. Cohen et a1.

[57] ABSTRACT Ultrathin membrane discs are prepared by spraying acasting solution of the membrane material onto a surface to form anumber of individual discs, allowing the solvent to evaporate from thediscs, and removing the discs from the casting surface. Slurries ofthese discs may be used to form dynamic membranes suitable for use inreverse osmosis applications such as the desalination of water.

9 Claims, 5 Drawing Figures PATENTEIJUCT 9 ms SHEET 2 UP. 2

(SMALLEST UN|T= IOpm) (SMALLEST UNIT= IO pm) DYNAMICS REVERSE OSMOSISMEMBRANES OF ULTRATHIN DISCS BACKGROUND Osmosis is a naturally occurringphenomenon in which pure solvent flows from a dilute solution to a moreconcentrated solution through a semipermeable membrane. The flow ofwater continues until a particular pressure difference across themembrane is reached, this pressure difference being the characteristicosmotic pressure of the system. By applying a pressure greater thanosmotic to the concentrated solution, the flow of water through thesemipermeable membrane can be reversed and pure water will flow from theconcentrated to the dilute solution. Unlike osmosis which tends to mixdilute and concentrated solutions, reverse osmosis can separate solventand solute. By applying pressure to a concentrated solution in contactwith the appropriate semipermeable membrane, pure solvent may berecoveredfrom the opposite side. Thus, reverse osmosis has been appliedwith some success to effect various separations, and particularly it hasevoked considerable interest for the desalination of water.

The efficacy of separation by reverse osmosis seems inseparably tied tothe quality of the semipermeable membrane used. The osmotic propertieswhich are most often employed as an indication of the capability of themembrane are the selectivity and the solvent flux. Selectivity is anindication of the ability of the membrane to distinguish between solventand soluteits relative ability to prevent the passage of solute whilepermitting the passage of solvent. The usual term indicative of theselectivity is the percentage salt rejection which is defined as 100times the difference in the concentration of the solute in the feedstream and in the permeate divided by the concentration of the feedstream. The other important property of the membrane is the flux whichis the rate of flow of permeatev through a unit area of the membrane. Itis generally indicated in units of gallons persquare foot per day.

A membrane which has acceptable osmotic properties will be uneconomicalunless it can maintain these properties for a reasonable length of time.Unfortunately, experience has shown that reverse osmosis membranes aresusceptible to several factors which cause a loss in performance.Chemical and/or biological degradation of the membranemay occur as wellas fouling of the membrane surface by impurities in the water. Inaddition, the structure of many conventional membranes is compacted bythe high pressures used in desalination by reverse osmosis causing anappreciable loss in the flux and product recovery. Other mechanicalfailures of the membranes such as leaks or cracks lead to appreciableflow of brine through the membrane and contamination of the productwater. When the reverse osmosis membrane has become less effective forone of these reasons, replacement may be required. This is extremelyexpensive since the dismantling of the high pressure equipment and thereplacement of the large amounts of membrane'surface requiresconsiderable amounts of time.

Several methods have been applied to solve these difficulties by formingthe membrane in situ. One method is shown in U.S. Pat. No. 3,592,763 inwhich membranes are formed in tubular porous supports by forcing asolution of the membrane through the tube with Lil a gas column. Themembrane is formed on the inside walls and is perfected by pumping air,hot water, etc., through the tube. When the performance of the membranebecomes uneconomical, the old membrane can be dissolved and a newmembrane formed in situ, thereby, saving the cost of dismantling theequipment and physically replacing the membranes.

Another method which has been used to form membranes in situ is to passa suspension of discrete particles over a porous supporting cloth, todeposit a layer of the particles on the support and to maintain thelayer with the pressure used in reverse osmosis separation. The dynamicmembrane, as it is called, has several advantages over the continuousmembrane formed in situ: first, it is easier to remove a dynamicmembrane merely by controlling the pressure and flow of water, andsecondly, the membranes can be repaired by passing a suspension of theparticles through the feed-side of the separation cell to patch anyleaks in the particle layer.

Dynamic membranes of various materials are shown, for example, in US.Pat. Nos. 3,503,789, 3,344,928, and 3,4 13,2l9. Only two patentssuggest, however, that dynamic membranes can be formed of neutralorganic polymers especially those commonly employed in the formation ofconventional membranes, for example, cellulose acetate. US. Pat. No.3,537,988 mentions that neutral organic polymers may be used as amembrane-forming additive. No indication, however, is given of themanner in which these materials could be employed as an additive.Similarly, US. Pat. No. 3,331,722 mentions that water solvatingmaterials such as cellulose acetate, cellulose propionate etc. may beused to form dynamic membranes. The materials are employed in the formof numerous discrete particles with an average diameter greater thanabout 0. l0 micron and preferably greater than 1.0 micron. Although thepatent mentions that these particles may be formed by any size reductiontechnique, there is no indication of how particles of cellulose acetateand similar polymers could be prepared.

The present invention is aimed at a method of preparing dynamicmembranes from ultrathin membrane discs formed from any conventionalmembrane material.

It is another object of my invention to form ultrathin I membrane discsfrom membrane materials, each disc exhibiting osmotic and physicalproperties which make it admirably suited for the formation of dynamicmembranes.

It is a further object of my invention to provide a reverse osmosisprocess which utilizes a membrane comprising a layer of ultrathinmembrane discs.

THE lNVENTlON l have now discovered a method by which improved dynamicmembranes suitable for reverse osmosis may be prepared. A significantfeature of the invention is the discovery that dynamic membranes can beformed from suspensions of ultrathin membrane discs which are inthemselves tiny reverse osmosis membranes.

l have found that these ultrathin membrane discs may be prepared from asolution of a conventional membrane material such as cellulose acetateby spraying the solution at controlled conditions onto an appropriatecasting surface. Numerous droplets are formed on the casting surface andthe evaporation of the solvent from these droplets forms the membranes.After appropriate post casting treatments a dilute suspension of thesemembranes discs can be prepared and used in the formation of dynamicreverse osmosis membranes.

The accompanying drawings illustrate several aspects of my invention.FIG. I shows the process of forming a dynamic membrane from a dispersionof ultrathin membrane discs, FIG. 2 is an illustration of this dynamicmembrane, and FIG. 3 shows the removal of the dynamic membrane.

FIGS. 4 and 5 are photomicrographs of ultrathin membrane discs.

DETAILED DESCRIPTION OF THE INVENTION An important aspect of myinvention is the discovery and development of a method for makingultrathin membrane discs. The essence of this process is the spraying ofa solution of a membrane material onto a casting surface to form anumber of small discs which after evaporation of the solvent aresemipermeable membranes. In this way membranes can be produced whichhave a diameter of approximately to 1,000 microns and are about 0.0l to0.1 micron thick.

One of the advantages of this invention is that a wider choice ofmembrane-forming materials can be used than has previously been the casewith continuous asymmetric membrane technology. For example, it isfeasible to use polymers which posses good intrinsic osmoticcharacteristics, but which are either very expensive or not amenable tofabrication into pressure stable asymmetric membranes. Numerousmaterials may be used to form the membrane discs. In particular, theinvention may be used to form ultrathin membrane discs from solutions ormelts of neutral polymers such as cellulose acetate, cellulose nitrate,cellulose butyrate, polyamides, poly-vinyl carbonate and similarcompounds.

A dilute solution of this membrane material is then sprayed on a castingsurface. In this step there are a number of variables which determinewhether discs will be formed and the quality of the discs formed. Amongthese variables are the relative speed of the spraying nozzle to thecasting surface, the distance of the nozzle from the casting surface,and the nozzle pressure. It is difficult to specify conditions whichwill give the best results for membrane production; in any situationthese operating conditions must be discovered through trial and error.From experience to date, however, the rough guidelines shown in Table Ican be used.

TABLE I Condition Broad Range Preferred Range Relative speed of 2 to 12ft/sec 8 ft/sec nozzle to casting surface Distance of nozzle 8 to 18inches 1 l to 14 inches from ca'sting surface Nozzle pressure 60 to I50psi 60 to I00 psi When selecting the proper parameters the distance ofthe nozzle from the casting surface and the relative velocity of thsedevices should be chosen so that regularly shaped individual dropletsare formed. If these conditions are not properly controlled, thedroplets may ei ther coalesce or take on irregular shapes which woulddetract from their ability to form dynamic membranes. The nozzlepressure is adjusted to control the size distribution of the discsformed. Depending on the type of spray nozzle used, the pressureemployed may make a considerable difference in both the average diameterand the size distribution of the discs formed.

In addition to these variables there are other conditions which mayaffect the membrane properties. Among these are the concentration of thecasting solution and the spray casting conditions such as the rate ofsolvent removal and the humidity. These variables will also be importantin determining the thickness and structure of the membrane discs and theosmotic properties they will have. The effects of variations in theseparameters may be predicted from the knowledge of their effect onconventionally cast membranes. For example, it has been found thatdilution of the casting'solution produces discs which are thinner andmore fragile than those produced from a more concentrated solution-aphenomenon also observed in casting continuous membranes.

The conditions which are most critical to the preparation of highquality membrane discs, however, are the solvent in the casting solutionand the spraying surface. Experience to date with various solvents andcasting surfaces indicate that production of suitable membrane discsdepends heavily on what happens when the droplet strikes the castingsurface and begins to form a membrane. The interaction of the castingsurface and the solvent are important to this process.

Two specific problems in the formation process have been observed.First, it has been noted that in some cases the first droplet ordroplets which are sprayed onto the casting surface spread out in a thinfilm over a large area. As subsequent drops strike the thin film,perfect membrane discs may be formed. Unfortunately, any attempt toremove the discs entangles them in the underlying polymer film and theybecome deformed and unusuable. This problem is particularly prevalentwhere the casting surface is a liquid or a liquid coated surface, but ishas also been observed when using dry casting surfaces. The best way tocombat this problem is to choose a solvent for the casting solutionwhich is compatible with the particular casting surface. Alternatively,the casting surface may be wetted with a thin film ofa liquid which willnot react with the casting solution and on which the casting solutionwill not spread.

The second problem which has been encountered involves the removal ofthe membrane discs from the casting surface. It is important that thedroplet be given sufficient time to form a disc which will not bedistorted by the removal process. Conversely, particularly where drysurfaces are used, the membrane discs must not be allowed to remain solong that they stick to the surface.

In short, this problem is a matter of finding the appropriate time forremovalof the membranes from the casting surface easily and without harmto the discs.

The spraying can be conducted in a variety of ways and on a number ofdifferent casting surfaces. The simplest method is to spray the castingsolution on a dry surface and after an appropriate time immerse thesurface or wash it with a liquid which will remove the discs. Although anumber of smooth materials, for example, chromium, Teflon, and glass maybe suitable for this process, to date the most successful of these hasbeen glass. When the appropriate casting solution has been used,ultrathin membrane discs sprayed on glass have been found to beperfectly formed and are easy to remove.

In addition to solid casting surfaces spraying may be performed onquiescent liquid surfaces. The problem of forming a base film upon whichsubsequent discs are formed, however, is a particular problem when aliquid casting surface is used. As mentioned previously, this problemmay be avoided by selecting a casting liquid and casting solvent whichdo not react in this manner. Alternatively the liquid casting surfacemay be pretreated with the solvent in the casting solution so thatspreading of the first drops of casting solution will not occur.

A third type of casting surface which may be used is a liquid coatedsolid surface. It is particularly advantageous in overcoming problems ofdisc removal and may be adapted to provide a continuous mechanizedprocess for preparing large quantities of ultrathin membrane discs. Forexample, a wetted rotating drum or belt may be used. The castingsolution is sprayed through a narrow slit to localize the spray and toprevent over-spraying. The system is designed so that the drum or beltrotates through a liquid bath or washing area to remove the discs at theappropriate time. A newly wetted surface leaves the bath or spray androtates back to the spray casting area to provide a fresh surface forthe casting of more discs.

After the membrane discs have been prepared by any of the basic castingtechniques, it is possible to improve their osmotic properties byappropriate treatment, for example, immersion in cold water or annealingin hot water. It is also possible that the membranes can be dried, forexample, by freeze drying process of Robert L. Riley et al., describedin US. Pat. No. 3,428,584. Appropriately dried membranes can be storedin the absence of water without any harmful effects. Later they can beprimed with water and are then ready for use.

To utilize the membrane discs to form a dynamic membrane the discs areslurried in water and are pumped into the feed portion of a reverseosmosis cell such as that shown schematically in FIG. 1. Although shownas a plate type reverse osmosis unit, any conven-v tional design ofreverse osmosis equipment can be used including the popular multipletube type which has the advantage of providing a large amount ofmembrane surface area in a given volume of separation equipment.

Referring to FIG. 1, a slurry of small membrane discs 7 are pumpedthrough line 1 into pressure vessel 3 which is separated into a feedchamber 4 and a product recovery chamber 5 by the porous support layer 2which transverses the vessel. The porous support may be any of thematerials conventionally used in the art such as cellulose acetate,polysulfone, sintered metal, porous glass, etc.

To form the dynamic membrane the reject valve 9 on line 8 leading fromthe feed side of the pressure vessel is closed so that the only avenuefor water to escape is through the porous support 2, into the productrecovery chamber 5 and out through valve 11 and line 10. By using thisprocedure it is unnecessary to gradually increase the pressure in orderto deposit a layer of membrane discs on the support layer 2. A constantpressure of about 800 psi is used and the flow of water through thesupport causes a suction force which as depicted in FIG. 1 leaves alayer 6 of overlapping membrane discs on the support. The completion ofthe layer can be detected by monitoring the flow rate of water comingthrough line 10. When the rate becomes constant, the

deposition of discs is no longer taking place to any great extent.

FIG. 2 is a perspective view of the dynamic membrane composed of poroussupport layer 2 and a layer of membrane discs 6. This layer 6 iscomposed of overlapping membrane discs 7 which effectively cover thesurface area of the porous substrate 2 and provide the active layerwhich is responsible for the salt rejection of the membrane. When usedin a reverse osmosis separation such as the desalination of water, theflow of the slurry of membrane discs is discontinued and saline water ispumped through line 1 into the feed chamber 4 at a pressure greater thanosmotic. Pure water passes through the layer of membrane discs 6 andporous substrate 2 into the product recovery chamber 5. Valve 11 is openand pure water is recovered through line 10. Valve 9 is also open sothat concentrated brine can be withdrawn from the .feed chamber via line8.

One advantage of this invention is that imperfections which develop inthe active membrane layer 6 during the course of operation may berepaired in situ. This is accomplished simply by following the procedureoutlined previously with regard to FIG. 1 using a dispersion ofultrathinmemb'rane discs. Otherwise, desalination will continueuninterrupted until the active membrane layer becomes clogged bydeposits or loses its salt rejecting capacity. At that time the existinglayer of membrane discs can be removed and replaced 'with a new layer,the entire operation being performed in situ.

The process of removing the membrane discs is shown schematically inFIG. 3. Water, either fresh or saline, is pumped at a low pressure andhigh flow rate through line 1 into the desalination unit 3. Since valve11 is closed, water does not flow through the membrane composed ofporous substrate 2 and a layer 6 of membrane discs 7 and into theproduct recovery compartment 5. Instead, the water flows across thesurface of the membrane in chamber 4, the turbulence disrupting themembrane disc layer 6 and sweeping these individual discs 7 throughvalve 9 and out line 8. If the turbulence of the sweeping stream isinsufficient to disrupt the layer of membrane discs, a solvent can beused which dissolves the membrane discs but does not affect the poroussupport layer. Where cellulose acetate membrane discs are used, removalcan be accomplished by hydrolysis with dilute aqueous ammonia.

Finally, although the description of my invention has been particularlydirected toward the reverse osmosis desalination of sea water, theultrathin membrane discs made by my invention may also be useful inpreparing dynamic membranes for use in the separation of water fromelectrolyte solutions including sea water, brackish water, acid minewater, and industrial brines and bitterns. The membranes may also beused in the separation of organic liquids, the purification andconcentration of liquid foods such as citrus juices, beer, and syrups;and the purification of liquid wastes such as urine. Example 1 Ultrathinmembrane discs were prepared from 1 percent solutions of celluloseacetate by spraying the solution onto a glass side. Spraying wasaccomplished using a nozzle with a conical spray pattern (nozzle No. 60,Spraying Systems, Company, Wheaton, Illinois) at a nozzle pressure ofpsi. The height of the nozzle above the casting surface casting distancewas varied and the translational speed of the nozzle relative to thesurface was chosen from among the alternatives of 2,

4 or 8 ft/sec. The degree and quality of disc formation was thendetermined by microscopic inspection. Some of the conditions at whichdiscs were successfully formed are shown in Table II. Several otherexamples are shown in which slight variations in these conditions failedto produce ultrathin membrane discs. Example 2 ln Example l membranediscs were prepared using glass as the spraying surface. Anothersuitable surface is wetted chromed plate which has been prepared byflaming the chromed surface and subsequently immersing it in water.Ultrathin membrane discs were prepared by spraying 1 percent solutionsof cellulose acetate onto this wetted chromed surface and subsequentlyremoving the discs with warm (50C) deionized water. A nozzle with a flatspray (nozzle No. 650017, Spraying Systems Company, Wheaton, lllinois)was used at a pressure of 100 psi, a height of 12.5 inches above thesurface and a translational speed of about 4 ft/sec. Solutions andconditions yielding well shaped discs are indicated in Table III. Thequality of the discs formed was determined by microscopic examination,and the percentage by weight of the ultrathin membrane discs which couldbe removed from the surface by washing was calculated.

lutions from which ultrathin reverse osmosis membrane discs weresuccessfully prepared. Example 4 Ultrathin membrane discs were preparedfrom solutions of 1 percent cellulose acetate in 4:l dioxane:methylethyl ketone or 1:1 dioxaneztetrahydrofuran. The solutions weresprayed through the same nozzle as that used in Example 2, at a pressureof 100 psi, a distance of ll inches from the dry glass casting surfaceand a translational speed of 4 ft/sec. Excellent discs were preparedfrom .either solution and could be readily removed by immersing theglass in water. Photomicrographs of these membrane discs were prepared.FIG. 4 is a photomicrograph of ultrathin discs prepared from thesolution of 4:1 dioxanezmethylethyl ketone.

FIG. 5 shows discs prepared from the 1 percent solution of celluloseacetate in 1:1 dioxaneztetrahydrofuran. In each drawing the smallestdivision represents 10 millimicrons. Example 5 Ultrathin membrane discswere prepared from a l percent solution of cellulose acetate in lzldioxane:tetrahydrofuran as in the previous example. The glass castingsurface was immersed in water seconds after spraying to removethe discs,cleaned, and resprayed TABLE ll Casting Casting speed distanceDescription of the Test Solvent system (ft/sec) (inches) spray-castparticles A I Dioxane 8 l8 Discs. structured center. B Do. l l 24 Somediscs. plus flat irregular shapes. C lzl Dioxane: 8 18 Disc. structuredcenter.

. .LLZ Trichlorpethanc W D Do. 2 l8 Polymer coalesced.

E 4: l Dioxane: 8 18 Fair discs, structured centers.

Methylethyl ketone F Do. 8 l l, 8 Tendency to coalesce. 0 Do. 2 l4Polymer coalesced. H Do. 4 l4 Some discs, also some irregular shapes. IDo. 8 l4 Discs. structured centers.

TABLE lll Percentage removal by Test Solvent system Results washing A4:] Dioxane: methylethyl ketone Good discs easily removed from plate. Byg l o lw xan qne Flat, slightly irregular shape 90 C 3: l Ethylacetatezacetic anhydride. F] 1 slightly ff-r u d ha 80 D 2: 1 Ethylacetate:acetic anhydride Do. 80 E l l Ethyl acetatezacetic anhydride Do.80 F l :2 Ethyl acetatezacetic anhydride Do. 80 G l :3 Ethylacetatezacetic anhydride Do. 80

Example 3 with the solution to form new discs. When a large quan- Tofurther compare the effects ofthe casting surface on the formation ofultrathin membrane discs. l percent cellulose acetate solutions ofvarious solvents were sprayed onto both a dry and a wetted glasssurface. The nozzle used in the previous example was used at a pressureof 100 psi, a distance of l 1 inches from the casting surface and atranslational speed of 4 ft/sec. Again the quality of the membrane discsformed was determined by observation with a microscope. Table IV liststhe sotity of the discs had been prepared, experiments were performed todetermine their suitability for reverse osmosis.

Tests were carried out using a 3 in. in diameter flat reverse osmosiscell. The substratum used was Millipore VSWP. The periphery of thesubstratum was masked with pressure sensitive tape to minimize theboundary effects in determining the ability to form dynamic membranes.After taping, an area of 14 cm TABLE IV Dry glass microscope slidedescription of the Appearance of particles obtained by spraying onto awetted Test Solvent system spray-cast panicle glass microscope slide A4: l Nitroethnnc:mcthanol Fair quality discs. wrinkled tippctir- (loudqtmlity discs. sllghtlygruiny ancc; 150-250 millimicron dia. appearance;140 millimicron dia. B l l Nitroethane methanol Fair quality discs.structured appear- Good quality discs. slightly-grainy ance; 100-150millimicron dia. appearance: 70-150 millimicron dia. C Nitroethane Fairdiscs. speckled" in appear- Good quality discs; about 70 ance; about50-250 millimicron dia. millimicron dia. I D 4: l Nitroethanezn-butanolGood quality discs. essentially Excellent discs; essentially featurelessinten'or; 50-180 millifeatureless;

micron dia. 40-200 millimicron dia. E 4: l Nitroethanetethyl acetatePoor quality. highly structured Good quality discs. textured discs;100-200 millimicron dia. appearance; 120-250 millimicron dia. FTriacetin Discs plus flat irregular shapes. Flat, near-round particles.

unstructured: 50-300 millicron dia.

remained exposed for the in si'tu deposition of the ultrathin membranediscs. The channel above the substratum was 10 mil high.

Deposition of the membrane discs was accomplished by pumping a slurry ofdiscs into the test cell. Best results were achieved by closing the exitvalve from the feed side of the membrane cell as depicted in FIG. 1. Noadvantage was obtained by incrementally raising the pressure rather thanusing a constant pressure. Disc deposition was routinely carried out ata pressure of 800 psi. Deposition was permitted to occur until areasonably constant flux through the membrane was achieved indicatingthat the formation of a thin layer of discs on the substratum had beencompleted. A typical membrane tested at 800 psi using a 100 ppm aqueoussolution of Trypan Blue dye was found not to pass dye indicating thatthe discs had deposited to form a reasonably tight membrane.

Membranes were then testsd at 205C on 3,000 ppm solutions of sodiumsulfate and sodium chloride at a pressure of 800 psi and a feed flowrate of 300 ml/min. Results are given in Table V.

TABLE V Depo- Sodium Sodium Run sition Flux Sulfate Chloride Time (gfd)Rejection Rejection A 1.0 96.0 [8.8 [9.0 22.0 l6.7 81.7 34.2 2.0 48 23.514.5 74.0 24.4 50 C 23.0 l8.7 45.5 l0.4 72.6 26.0

Example 6 Tests were also performed to evaluate the formation ofdyanamic membranes in tubular reverse osmosis destraited. 100-Z50millimicron dia.

vices. The equipment used consisted of a one-half inch diameter outertube supporting a concentric substratum. Outer tubes of both braidedglass fiber and of stainless steel were utilized. The stainless steeltubes had 13 mil holes drilled in the test area for removal of permeate.The substratum consisted of a liner of Dacron-polysulfone. The objectivewas to form a dynamic membrane on the inside surface of this liner. Whensaline water is passed under pressure through the inside of the tube theactive layer separates out pure water which leaves through the poroussubstratum and the outer tube.

To minimize the entrance effects during the deposition of the membrane.the first 9 inches of the 13.5 inch long liner were dipped in a 7.5percent solution of ethyl cellulose in methanol. This provided aneffective sealent and isolated a small area in which membrane depositioncould be evaluated.

Ultrathin membrane discs were prepared by spraying a 1 percent solutionof cellulose acetate in 1:4 dioxanezmethylethyl ketone onto glassplates, followed by immersion in deionized water within 30 seconds toremove the discs. Deposition of the membrane on the liner was achievedby passing a dispersion of the membrane discs into the membrane tube ata pressure of 800 psi. The reject valve was opened briefly to removeentrapped air and was then closed during membrane deposition. Membraningwas considered complete when the flux approached a constant valve.Reverse osmosis properties were then evaluated at 25C on a 3.000 ppmsolution of either sodium sulfate or sodium chloride. The feed solutionpressurized at 800 psi was fed at the rate of either 1 or 2 gallons perminute. The results are given in Table VI.

In a second run the membrane discs were annealed at C prior todeposition. The slight increase in the salt retention with respect toeither solution indicates that reverse osmosis performance of thedynamic membrane is related to the intrinsic properties of the discs.

TABLE V1 Sodium Sodium Deposition Feed sulfate chloride time flow rateFlux rejection rejection Discs (hrs) Tube type (gal/min) (gfd) (7b) (%1Standard 89 /&-inch diameter braided 10.3 93.7 31.5

glass-fiber tube Annealed 47.5 16-inch diameter stainless 2 l3.5 96 33.5

steel tube I claim: 1. A membrane suitable for use in a reverse osmosisseparation consisting of:

an active layer comprising numerous overlapping ultrathin membrane discsand a porous support layer offering structural support for said activelayer and allowing the free passage of permeate away from said activelayer. 2. The membrane of claim 1 in which said reverse osmosisseparation is the desalination of water.

3. The membrane of claim 1 in which said ultrathin membrane discsconsist of a neutral polymer.

4. The membrane of claim 3 in which said neutra' polymer is celluloseacetate.

5. A process for the separation of pure solvent from a solution byreverse osmosis comprising the steps of: forming a dispersion ofultrathin membrane discs, passing said dispersion over the feed side ofa porous support material in the separation apparatus to deposit a layerof ultrathin discs on said support material,

using said layer of ultrathin membrane discs as the active reverseosmosis membrane to effect said separation, and

repairing at least a portion of said layer of ultrathin membrane discswhen necessary by passing said dispersion of ultrathin membrane discsover the feed side of said porous support material to remake said layer.

6. The process of claim 5 wherein said separation is the desalination ofwater.

7. The process of claim 5 wherein said ultrathin membrane discs arecomposed of a neutral polymer.

8. The process of claim 7 wherein said neutral polymer is celluloseacetate.

9. The process of claim 5 including the step of rem oving said activelayer of ultrathin membrane discs when replacement is required, prior tosaidrepairing of at least a portion of said layer.

2. The membrane of claim 1 in which said reverse osmosis separation isthe desalination of water.
 3. The membrane of claim 1 in which saidultrathin membrane discs consist of a neutral polymer.
 4. The membraneof claim 3 in which said neutra polymer is cellulose acetate.
 5. Aprocess for the separation of pure solvent from a solution by reverseosmosis comprising the steps of: forming a dispersion of ultrathinmembrane discs, passing said dispersion over the feed side of a poroussupport material in the separation apparatus to deposit a layer ofultrathin discs on said support material, using said layer of ultrathinmembrane discs as the active reverse osmosis membrane to effect saidseparation, and repairing at least a portion of said layer of ultrathinmembrane discs when necessary by passing said dispersion of ultrathinmembrane discs over the feed side of said porous support material toremake said layer.
 6. The process of claim 5 wherein said separation isthe desalination of water.
 7. The process of claim 5 wherein saidultrathin membrane discs are composed of a neutral polymer.
 8. Theprocess of claim 7 wherein said neutral polymer is cellulose acetate. 9.The process of claim 5 including the step of removing said active layerof ultrathin membrane discs when replacement is required, prior to saidrepairing of at least a portion of said layer.